Bash Basics

by Dan Aukes

Arguments

short arguments usually have a single dash followed by a single character. An example is ls -a, where -a is an additional flag/attribute that indicates to the ls command that you should list all files, even if hidden by a .

long arguments usually have two dashes followed by a word or phrase (connected by dashes). An example is ls --all, where --all has the same meaning as -a above.

Getting help

<command> -h: display help
<command> --help: display help (long form of argument)
man <command>
or you can go to https://explainshell.com

Most Common Commands

cd <absolute or relative path>: change directory to the path indicated
ls: list files in current directory
ls -la: list all files(a) in list format(-l)
chown <username>:<groupname> <file or folder>: change the owner of the file or folder
chown -R <username>:<groupname> <file or folder>: change the owner of the folder recursively
chmod <0-7><0-7><0-7> <file or folder>: change the owner:group:anyone permissions for a file
chmod -R <0-7><0-7><0-7> <file or folder>: change the owner:group:anyone permissions for a folder recursively
chmod +x <file or folder>: add execution permission to a file/folder
sudo+<command>
mkdir <path>: make a directory at path indicated
mv <path1> <path2>: move a file from first path indicated to second path indicated
cp <path1> <path2>: copy files from first path indicated to second path indicated
cp -r <path1> <path2>: copy files recursively from first path indicated to second path indicated
rmdir <path>: delete a folder at a given path (folder must be empty)
rm <path1> ...: delete a file at a given path(s)
rm -rf <path> ...: delete a file/folder at a given path(s) using the force and recursive option (removes folders too, be careful!) | *

Keystrokes

key combineation	meaning
`ctrl+c`	stops execution of whatever is running in bash
`ctrl`+`alt`+`t`	opens up a new `bash` terminal window
`ctrl`+`shift`+`t`	opens up a new `bash` tab
`ctrl`+`shift`+`w`	closes the current `bash` tab
`ctrl`+`shift`+`q`	closes all `bash` tabs

Common environment variables

$HOME
$USERNAME
$PWD
$USER
$PATH
$PYTHONPATH
printenv list all current environment variables
export will save an environment variable across sessions

Other common commands

chown
chmod
sudo
ps
kill
killall
htop
grep
pwd
printenv
export
cat
login
shutdown
reboot
diff
rclone
cd
mv
rsync
scp
ssh
echo
mkdir
rm
rmdir
cp
touch
ln
chgrp

sudo

to run something as root

sudo <command> [command options]

to run interactively as root

sudo -i

to run something as someone else

sudo -i -u <username>

run exit to leave that session

Find/Replace

To change the file in place:

sed -i "s/regex/replace/" file

sed -i "s|regex|replace|" file

To copy output to a new file

sed "s/regex/replace/" filein > fileout

Recursively chown

#chown -R <owner> <folder_1> <folder_2> ... <folder_n>
chown -R user /home/user

Misc

Count files in bash

ls -l1 | wc -l

octal file permissions

stat /path/to/filename

stat -c '%A %a %n' /path/to/filename

Print your environment variables

printenv | grep ROS

tail

see the last few lines

#tail -100 <filename>
tail -100 history.txt

see real-time changes to files as they get appended:

tail -f history.txt

see line 196-200 of a file

head -200 history.txt | tail -5

Recursively list files

tree /path/to/dir

Awk

awk '{print $1}' <filename>

cat <filename> | awk '{print $1}'

awk -F , '{print $1}' <filename.csv>

close bash without saving history

unset HISTFILE && exit

history -c && history -w && exit